Hydropneumatic suspension system

ABSTRACT

The invention pertains to a hydropneumatic suspension system, in particular for motor vehicles, with at least one hydraulic strut (2) that acts upon at least one hydropneumatic piston-type accumulator (6) during deflection and rebound movements. The piston-type accumulator (6) has a separating piston (22) that separates the hydraulic accumulator chamber (24) from a spring chamber (26) containing a compressible medium, in particular a gas. A hydraulic pressure (P h ) acts upon the separating piston (22) from the side of the accumulator chamber (24) and a pneumatic pressure (p p ) acts upon the separating piston from the side of the spring chamber (26), with at least one supplementary spring force (F F  ;F F1  ;F F2 ) acting upon the separating piston (22) of the piston-type accumulator (6) in addition to the forces (F h ,F p ) resulting from charging the separating piston (22) with the hydraulic pressure (P h ) and the pneumatic pressure (p p ). This supplementary spring force is generated by at least one spring element (28;34;36) that is arranged outside of the spring chamber (26).

The present invention pertains to a hydropneumatic suspension system, inparticular for motor vehicles, comprising at least one hydraulic strutthat during its deflection and rebound movements acts via a hydraulicmedium upon at least one hydropneumatic piston-type accumulator that hasa separating piston which separates a hydraulic accumulator chamber froma spring chamber that contains a compressible medium, in particular agas, with a hydraulic pressure that originates from the accumulatorchamber and a pneumatic pressure that originates from the spring chamberacting upon said separating piston, and with at least one supplementaryspring force acting upon the separating piston of the piston-typeaccumulator in addition to the forces resulting from charging theseparating piston with the hydraulic pressure and the pneumaticpressure.

In hydropneumatic suspension systems, a hydraulic medium is displacedduring the deflection and rebound movements of the strut, i.e., due tomovement of a piston inside of a cylinder. When deflecting the strut, acertain volume of the hydraulic medium flows into the accumulatorchamber of a hydropneumatic accumulator such that the volume of acompressible medium, usually a gas, that is contained in saidhydropneumatic accumulator is reduced. This compression causes apressure increase and consequently an elastic spring effect thatsubsequently causes a backflow of the hydraulic medium for the reboundmovement of the strut. In a static position of the strut, the pneumaticpressure generates via the hydraulic medium a supporting force forcarrying the respective load inside the strut.

The essential advantage of hydropneumatic suspension systems can be seenin the fact that nearly constant suspension characteristics can beattained even under changing load ratios (ratio between the empty loadand the full load of the strut) due to the fact that the level of themotor vehicle and consequently also the available travel of the strutcan practically be maintained constant by means of a hydraulic levelingprocess, i.e., by introducing or removing the hydraulic medium. Thismeans that it is practically possible to attain a suspension that doesnot depend on the load. Consequently, systems of this type are primarilysuitable for motor trucks in which a high load ratio usually occurs.

However, this high load ratio is associated with the disadvantage thatit requires that the compressible medium has a very large total volumesuch that the entire load ratio of the strut can be absorbed by thespring chamber or the medium contained therein while causing thesmallest possible pressure changes (volume changes of the compressiblemedium) and--for reasons of comfort--attaining a flat, "smooth"characteristic line. However, this requires a very large structural sizeof the spring chamber with a long displacement path of the separatingpiston. This can cause installation problems in a motor vehicle andnaturally is also associated with excessive costs regarding theaccumulator.

U.S. Pat. No. A 4,153,237 discloses a suspension system of the initiallymentioned type in which a helical pressure spring which charges theseparating piston with a supplementary spring force that acts in thedirection of the pneumatic pressure and opposite to the hydraulicpressure is arranged inside of the spring chamber, i.e., inside of thepneumatic pressure medium, in order to influence the springcharacteristics of the piston-type accumulator. However, this isassociated with the disadvantage that the helical spring substantiallylimits the range of travel of the separating piston in the direction ofthe spring chamber such that the pneumatic medium cannot be compressedarbitrarily. Consequently, the spring characteristics cannot be easilyadjusted to arbitrary requirements.

European Patent No. A 0,052,782 (or the corresponding U.S. Pat. No. A4,478,431) discloses one additional suspension system in which at leasttwo accumulators are provided for each strut. In a few of the respectiveembodiments, one of the accumulators provided is designed as apiston-type accumulator, the separating piston of which is additionallycharged by a spring that is arranged in the gas chamber. The adaptationof the spring characteristic is quite difficult due to the at least tworespective accumulators provided.

In a suspension system disclosed in German Patent No. A 3,936,034, it isproposed to design the piston-type accumulator as a pressure transducer,namely in such a way that the pneumatic pressure of the compressiblemedium in the spring chamber is always less intense than the hydraulicpressure inside of the accumulator chamber as well as inside of thestrut. According to one additional development of this invention, themovement of the piston inside of the cylinder of the strut should betransferred onto the separating piston via the displaced hydraulicmedium (intensifying of the hydraulic pressure) with a certain pathincreasing or reducing the ratio.

One additional hydropneumatic suspension system is disclosed in GermanPatent No. A 4,008,831. According to this publication, the piston of thestrut divides two pressure chambers inside of the cylinder by means ofits piston seal, namely an annular chamber that encloses a piston rodand a "load bearing" cylinder chamber that is situated opposite to thepiston rod, with both of these pressure chambers being hydraulicallyconnected with one respective, separate spring chamber independentlyfrom one another. One spring chamber produces a first hydraulic pressureinside of the cylinder chamber which generates a supporting force(carrying force) by charging the corresponding pressure surface of thepiston, with the other spring chamber charging the annular chamber andthe pressure surface of the piston that faces said annular chamber witha second hydraulic pressure such that a force that acts opposite to thesupporting force is generated. In this known suspension system, the loadratio to be absorbed by the "load bearing" spring chamber is reduced bythis counter force that acts as an "artificial load" due to the factthat said counter force always acts in addition to the respective loadthat is actually present. A load ratio of, for example, 1:10 wouldresult without this counter force at an empty load of, for example, 6 kNand a full load of, for example, 60 kN. In this case, a load ratio of 16kN:70 kN=1:approximately 4.4 would be obtained if a counter force of,for example, 10 kN that acts in the load direction would be generated.Although this measure allows a reduction of the structural size of theload bearing spring chamber or its required gas volume, the fact that atotal of two spring chambers (per strut) are required represents acertain disadvantage since a spring chamber is a relatively expensivecomponent due to its construction design, in particular the requiredseals. In addition, sealing problems within the region of the pistonseal of the strut may occur because said piston seal is charged with therespective hydraulic pressures from both sides. These two hydraulicpressures usually differ slightly such that the piston seal only needsto provide a seal against a small differential pressure. However, inorder to provide a reliable seal, a piston seal needs to be charged witha certain minimum pressure on one side. This is the reason why leaksmight occur inside of the strut of this known suspension system due tothe fact that the hydraulic medium is transferred from the cylinderchamber into the annular chamber or vice versa via the piston seal suchthat an undesired change of the spring characteristic results. Inaddition, the generation of the counter force within the region of thestrut also causes the undesirable side effect that, in the case of aleveling process in which the strut is, for example, moved to a nominalstatic level (usually approximately in the central position of itstravel) by introducing the hydraulic medium during a load increase, therespective counter force increases since a raising of the level isalways associated with a volume reduction of the annular chamber of thestrut and consequently with a pressure increase in the correspondingchamber. Consequently, the device that generates the hydraulic pressure,usually a pump, needs to be very powerful; this means that a pumpcapacity in excess of 200 bar, e.g., between 230 and 250 bar, can berequired in motor trucks.

The present invention is based on the objective of simplifying asuspension system of this type with respect to its construction,reducing the financial expenditures for its components and improvingsaid suspension system in such a way that an arbitrary and constant,optimal spring characteristic can be attained with simple means.

According to the invention, this objective is attained with thecharacteristics disclosed in claim 1. Advantageous developments andspecial embodiments of the invention are described in the dependentclaims.

The invention makes it possible to compress the compressible mediuminside of the spring chamber almost arbitrarily, which is the reason whyan adjustment of the resulting spring characteristic within a very broadrange is possible in association with charging the separating pistonwith the supplementary spring force. In addition, the invention providessubstantial advantages as compared to the state of the art, inparticular U.S. Pat. No. A 4,153,237; these advantages will be discussedin detail in the following description.

The effective supplementary spring force influences the volume of thecompressible medium--in addition to the "normal" volume changes causedby the loads and the spring travel--in such a way that the total volumeof said compressible medium can be advantageously reduced. The reasonfor this can be seen in the fact that the supplementary spring forceadjusts a load ratio (or a compression and pressure ratio that-deviatesfrom the "normal" conditions that exist without the supplementary springforce) that differs from the load ratio of the strut for the piston-typeaccumulator or its spring chamber, with this "accumulator load ratio"being substantially smaller than the load ratio of the strut. This meansthat the entire accumulator can be constructed in a more compact fashionsuch that a shorter travel of the separating piston is attained.

Due to the aforementioned reasons, the disadvantageous generation of acounter force within the region of the strut as it is known from GermanPatent No. A 4,008,831 becomes unnecessary. During the leveling process,the pump only needs to generate a pressure that corresponds with theactual load, so that a maximum pump capacity of 200 bar suffices in allinstances. In addition, only one accumulator per strut is required. Thisis advantageous because the same hydraulic pressure is present in thetwo pressure chambers of the strut, so that the aforementioned sealingproblems are eliminated. Alternatively, it is also possible to connectthe annular chamber with the atmosphere via a ventilation opening, sothat the aforementioned sealing problems are also eliminated in thisinstance because the piston seal is able to provide a seal against ahigh differential pressure.

In a first embodiment of the invention, the supplementary spring forcecharges the separating piston in the direction of the spring chamber,i.e., in the direction of the "hydraulic force," namely in such a waythat the supplementary spring force is more intense at a small load ofthe strut and a correspondingly less intense hydraulic pressure than ata higher load and a correspondingly higher hydraulic pressure. Thismeans that the supplementary spring force according to the inventionadditionally compresses (reduces the volume of) the compressible mediumin an "artificial" fashion, i.e., in addition to the normal load, withsaid "artificial compression" being relatively more intense at a smallload than at a high load. In this embodiment, the effective direction ofthe supplementary spring force extends exactly opposite to the oneaccording to the state of the art (U.S. Pat. Nos. A 4,153,237, A4,478,431), so that the spring element may, for example, be arrangedinside of the accumulator chamber in the form of a pressure spring so asto realize the arrangement of the spring element "outside of the springchamber" according to the invention.

According to a second embodiment, the supplementary spring force chargesthe separating piston in the direction of the accumulator chamber, i.e.,in the direction of the "pneumatic force," namely in such a way that thesupplementary spring force is less intense at a smaller load of thestrut and a correspondingly smaller hydraulic pressure than at a higherload and a correspondingly higher pressure. Consequently, this pertainsto an "artificial expansion" (volume increase) of the compressiblemedium as compared to the conditions under a "normal load," with this"artificial expansion" being relatively more intense at a high load thanat a small load. This effective direction essentially corresponds withthe state of the art (compare in particular to U.S. Pat. No. A4,153,237), but the spring element according to the invention isarranged "outside of the spring chamber." This is preferably realized byproviding the separating piston with a sealed separating piston rod thatextends out of the piston-type accumulator, with at least one springelement engaging said separating piston rod so as to be able to chargethe separating piston with the supplementary spring force via theseparating piston rod. This advantageous measure naturally may also beapplied in the first embodiment if the supplementary spring force actsin the same direction as the hydraulic force, i.e., opposite to thepneumatic force.

According to one particularly advantageous development of the invention(third embodiment), the separating piston is charged with one respectivesupplementary spring force in both directions, namely by means ofexternal spring elements via a separating piston rod of the separatingpiston. In this case, a resulting supplementary spring force is obtainedfrom the sum (difference) of these two spring forces that act inopposite directions, namely in such a way that said supplementary springforce reduces the pneumatic pressure that corresponds with a high loadof the strut and increases the pneumatic pressure that corresponds witha small load of the strut. Due to this preferred measure, the volume ofthe compressible medium required for the respective load ratio of thestrut is reduced in a particularly effective fashion as elucidated belowin a simple numerical example.

At a given load ratio and design of the suspension system that is hotprovided with the measures according to the invention, the pressure in aspring chamber with a volume of 1000 cm³ amounts to, for example, 150bar in the loaded condition and, for example, 40 bar in the emptycondition. The pressure ratio 150 bar:40 bar results in a factor of 3.75that must be multiplied with the volume in the loaded condition ("loadvolume") in order to obtain the volume in the empty condition ("emptyvolume"); this means that an empty volume of 1000 cm³ ×3.75=3.750 cm³ isrequired in the empty condition.

However, the supplementary spring force according to the inventionreduces the "loaded pressure" to a value of, for example, 100 bar andincreases the "empty pressure" to a value of, for example, 60 bar suchthat a ratio factor of 100:60=1.667 is obtained. At the aforementioned"load volume" of 1000 cm³, this results in a reduction of the "emptyvolume" to only 1000 cm³ ×1.667=1667 cm³. The volume of the compressiblemedium required for the load ratio of the strut consequently may besubstantially reduced with the measures according to the invention,namely by more than 50% in the previously described example.

The reduction of the gas volume in the empty condition also causes thespring stiffness c to become more intense in the empty condition, sothat the natural frequency of the system also remains practicallyidentical in the loaded condition and in the empty condition. Incomparison to a large "empty gas volume" that existed most of the timein customary systems, a large difference between the natural frequencyin the loaded condition and the empty condition is always obtained dueto less intense spring stiffness.

Due to the process of influencing the pressure ratios inside of thepiston-type accumulator by means of the supplementary spring force, itmay--in dependence on the respective design of system parameters (inparticular the resulting characteristic line of the supplementary springforce)--occur that the pneumatic pressure becomes more intense than thehydraulic pressure beginning at a certain load. In this case, theseparating piston needs to be equipped with a special "gas seal."Naturally, the system also may be designed in such a way that thepneumatic pressure is always less intense or no more than identical tothe hydraulic pressure, so that a simpler "oil seal" suffices for theseparating piston.

The supplementary spring force according to the invention can begenerated by different means, e.g., with mechanical pressure and/ortension springs or with hydropneumatic springs (accumulators). A fewadvantageous possibilities are described in detail in the followingdescription of preferred embodiments.

The invention provides substantial advantages as compared to the stateof the art. In particular the preferred measure of allowing a springelement to act upon the separating piston via a separating piston rodthat extends out of the accumulator represents a considerabletechnological advance because the spring element that charges theseparating piston(s) is arranged externally, i.e., outside of theaccumulator, as compared to an internal arrangement of a mechanicalspring in the gas chamber--e.g., in accordance with U.S. Pat. No. A4,153,237. Among other things, this measure provides the followingimportant advantages:

1. If one intends to alter the spring characteristics, the springelement(s) may be exchanged simply without having to disassemble theaccumulator.

If the spring were inside of the gas chamber, the accumulator would haveto be disassembled in order to exchange each spring and reassembled witha high sealing cost after the spring is exchanged. In addition, anexchange of the spring elements also stresses the sealing surfaces, sothat the accumulator may be rendered unusable after several exchanges ofthe spring (which might be required for adapting the springcharacteristics).

2. According to the invention, possible damages within the region of thespring element that charge the separating piston are easily detectablefrom the outside. A fracture of a mechanical spring is, for example,immediately visible; the broken spring can be easily replaced (compareto 1.).

In the known internal arrangement, a broken spring cannot be detectedimmediately, but different spring characteristics would be noticed forno apparent reason. The diagnosis indicating a broken spring can only bemade after a time-consuming and costly search for the defect and afterdisassembling the accumulator. Until this diagnosis is made, the innersurfaces of the accumulator undoubtedly would already be damaged by thesharp edges within the region of the fracture of the spring such thatthe accumulator would be rendered unusable.

3. In order to adjust the spring characteristics, it is frequentlyrequired to alter the pneumatic pressure, with said fact inevitablycausing a change of the supplementary spring force that charges theseparating piston because said separating piston is displacedcorrespondingly during this process. This change in the spring forcecannot be compensated for if the spring element is internal.

According to the invention, the preferred external arrangement of thespring element(s) makes it possible to change this prestress viaadjusting means, so that a change in the spring prestress that is causedby a change of the pneumatic pressure (due to the displacement of theseparating piston) can be compensated for arbitrarily. In addition,tolerances caused by the manufacturing process of the spring elements(e.g., longitudinal tolerances) can be easily compensated for via theadjusting means.

A few advantageous embodiments of such adjusting means are described indetail in the following description.

The invention is described in detail below with reference to severalembodiments that are illustrated in the enclosed figures. The figuresshow:

FIG. 1: a first embodiment of the suspension system according to theinvention in the form of simplified sectional representations of itscomponents, in particular a strut and a piston-type accumulator,

FIG. 2: a view that is analogous to FIG. 1 of a second embodiment,

FIG. 3: one additional representation that is analogous to FIG. 1 of athird embodiment,

FIG. 4: the essential parts or components of a fourth embodiment of thesuspension system according to the invention,

FIG. 5: the piston-type accumulator of an additional preferredembodiment of the invention,

FIG. 6: a side view of an advantageous development of a piston-typeaccumulator in which the components are illustrated in the form of apartial axial section,

FIG. 7: a detailed partial axial section of the hydropneumaticpiston-type accumulator according to FIG. 6, and

FIG. 8: one additional possible embodiment of a suspension system.

In the different figures, identical components are always identified byidentical reference numerals, so that each description of a componentapplies to the other respective figures in which this component is alsoidentified by the corresponding reference number.

A suspension system according to the invention consists of at least onehydraulic strut 2 and a hydropneumatic piston-type accumulator 6 that isconnected with the aforementioned strut via a hydraulic connection 4.

The strut 2 consists of a cylinder 8, a piston 10 that moves inside ofthe aforementioned cylinder and a piston rod 12 that is connected withthe piston 10 and extends out of the cylinder 8, with said pistonseparating inside of the cylinder 8 a "load-bearing" cylinder chamber14, i.e., a cylinder chamber that is under a load-dependent hydraulicpressure p_(h), from an annular chamber 16 that encloses the piston rod12. In order to support the wheel or the axle of a motor vehicle, thestrut 2 is arranged between a non-sprung mass and a sprung mass, e.g.,by connecting the free end of the piston rod 12 with the wheel 18indicated in the figure or a certain part of the axle and connecting theopposite end of the cylinder 8 with the frame of the motor vehicle.Naturally, a "reversed" installation is also possible.

The piston-type accumulator 6 preferably is designed and arrangedspatially independently from the strut 2 and consists of an accumulatorcylinder 20, inside of which a separating piston 22 "floats," i.e., isguided in a freely movable fashion. The separating piston 20 dividesinside of the accumulator cylinder 20 a hydraulic accumulator chamber 24that is connected with the strut 2 via the hydraulic connection 4 from aspring chamber 26 that is filled with a compressible medium, inparticular a gas.

The load-dependent hydraulic pressure P_(h) present inside of thecylinder chamber 14 of the strut 2 also acts in the accumulator chamber24 of the piston-type accumulator 6 via the hydraulic connection 4. Apneumatic pressure p_(p) is adjusted at this location by displacing theseparating piston 22 correspondingly and compressing the compressiblemedium in the spring chamber 26. Consequently, the separating piston 22is charged with the hydraulic pressure p_(h) on its side that faces theaccumulator chamber 24 such that a "hydraulic force" F_(h) that has thetendency to displace the separating piston 22 in the direction of thespring chamber 26 is generated due to the relation F=p×A. However, a"pneumatic force" F_(p) that is generated by charging the oppositepressure surface of the separating piston 22 with the pneumatic pressurep_(p) acts opposite to the aforementioned hydraulic force.

According to the invention, it is proposed that the separating piston 22is--in addition to the hydraulic and pneumatic pressures p_(h), p_(p) aswell as the forces F_(h) and F_(p) resulting thereof--charged with asupplementary spring force F_(F). There exist several possibilities forcharging the separating piston with a supplementary spring force, with afew of these possibilities described below in an exemplary fashion withreference to the individual figures.

According to FIG. 1, the separating piston 22 is charged with asupplementary spring force F_(F) that acts in the direction of thespring chamber 26 (this supplementary spring force is indicated in FIG.1 by a continuous arrow). This supplementary spring force is generatedby a mechanical spring element 28 that is designed as a helical pressurespring and arranged inside of the accumulator chamber 24. According tothe invention, this measure makes it possible that the supplementaryspring force F_(F) is--due to the spring characteristics of the springelement 28--greater for a small load on strut 2, that is if theseparating piston 22 is situated in a position in which it is displacedin the direction of a final limit stop on the side of the accumulatorchamber 24 (small volume of the accumulator chamber and larger volume ofthe spring chamber) due to the correspondingly low hydraulic andpneumatic pressures p_(h) and p_(p) than at a higher load at which theseparating piston 22 is displaced further in the direction of the springchamber 26 due to the higher pressures p_(h) and p_(p). However, theequation F_(p) =F_(h) +F_(F) applies respectively.

One possible alternative to the embodiment according to FIG. 1 consistsof the fact that the supplementary spring force F_(F) (indicated by anarrow drawn with broken lines) charges the separating piston 22 in thedirection of the accumulator chamber 24. This may, for example, berealized by designing the spring element 28 in the accumulator chamber24 as a tension spring. This embodiment realizes an effect that is"opposite" to the aforementioned effect, i.e., the supplementary springforce F_(F) is less intense at a smaller load of the strut 2 andcorrespondingly less intense pressures p_(h), p_(p) than at a higherload and correspondingly higher pressures p_(h), p_(p). In this case,the equation F_(h) =F_(p) +F_(F) or F_(p) =F_(h) -F_(F) applies.

In the embodiment illustrated in FIG. 2, the separating piston 22 ischarged with a first spring force F_(F1) (same direction as thehydraulic force F_(h) ) as well as an opposing second spring forceF_(F2) (same direction as the pneumatic force F_(p)), so that theresulting supplementary spring F_(F) is obtained from the differenceF_(F) =F_(F1) -F_(F2). Depending on the intensity of the individualspring forces F_(F1) and F_(F2), this results in the separating piston22 being charged in the direction of the accumulator chamber 24 or inthe direction of the spring chamber 26. According to the invention, itis particularly advantageous that the spring characteristics can beadapted in such a way that the resulting supplementary spring forceF_(F) reduces the pneumatic pressure p_(p) that corresponds with a highload of the strut 2 (the force F_(F2) predominates, while said resultingsupplementary spring force increases the pressure p_(p) that correspondswith a small load (the force F_(F1), predominates). The effects of thismeasure was already described initially, so that we refer to theintroduction of the description for additional explanations.

FIG. 2 also shows that the separating piston 22 of this embodiment isdirectly charged via a separating piston rod 30 that is connected withsaid separating piston, with the separating piston rod 30 extendingthrough the accumulator chamber 24 and out of the accumulator cylinder20 in a sealed fashion. The free end of the separating piston rod 30 isconnected with a force transfer element 32 that is charged on one sideby a first spring element 28--that functionally corresponds with thespring element 28 in FIG. 1 and is identified by the same referencenumber--and charged in the opposite direction by a second spring element34. In the example shown, both spring elements 28 and 34 are designed ashelical pressure springs, but said spring elements may also be replacedby tension springs that function correspondingly. In addition, it ispossible to use a combination of pressure springs and tension springs.

FIGS. 3, 4 and 5 show embodiments in which a spring element 36 that actsupon the separating piston 22 via the separating piston rod 30 is formedby a cylinder/piston unit 38. This cylinder/piston unit 38 has at leastone pressure chamber 40 or 42, inside of which an elastic prestresspressure p_(v1) or P_(v2) is present such that the corresponding springforce F_(F) or F_(F1), F_(F2) is generated by charging a piston 44.

In the embodiment illustrated in FIG. 3, the pressure chamber 40 is onthe side of the piston 44 which is situated opposite to the separatingpiston 22 and charged with the pressure p_(v1), so that this embodimentfunctionally corresponds with the one illustrated in FIG. 1 because thesupplementary spring force F_(F) also charges the separating piston 22in the direction of the spring chamber 26, with said supplementaryspring force corresponding with the first spring force F_(F1).

In the embodiment according to FIG. 4, the pressure chamber 42 isarranged on the side of the piston 44 that faces the separating piston22 and charged with the pressure P_(v2), so that the supplementaryspring force F_(F) =F_(F2) charges the separating piston 22 in thedirection of the accumulator chamber 24.

The respective pressure chamber 40, 42 in the embodiments according toFIG. 3 and 4 is filled with a compressible medium, so that the pressurechambers 40, 42 respectively form pneumatic spring chambers.

The embodiments according to FIGS. 3 and 4 naturally may also becombined with one another in such a way that an embodiment thatfunctionally corresponds with the one shown in FIG. 2 is created, withthe piston/cylinder unit 38 comprising both pressure chambers 40 and 42;in this case, the following equation applies:

    F.sub.F =F.sub.F1 -F.sub.F2.

This particular embodiment is illustrated in FIG. 5. In this case, thepressure chambers 40 and 42 are not filled with a pneumatic medium, butrather with a hydraulic medium, with each pressure chamber 40, 42 beingconnected with a hydropneumatic accumulator 46,48.

According to the invention, it is particularly advantageous if theintensity of the respectively effective supplementary spring force F_(F)(F_(F1), F_(F2)) can be changed via an adjusting means. This may, forexample, be realized in the embodiment illustrated in FIG. 2 by changingthe prestress of at least one of the spring elements 28 and/or 34, e.g.,via mechanical adjusting elements 50 and/or 51. In the embodimentsaccording to FIGS. 3-5, at least one of the respective elastic prestresspressures P_(v1) and/or P_(v2) can be varied via suitable pressureadjusting means. This measure advantageously allows an adjustment of thecharacteristic line of the respectively effective supplementary springforce F_(F), in a relatively simple fashion and over a very broad range(large field of characteristic lines).

In addition, it is particularly advantageous for certain instances thatthe respectively effective or resulting supplementary spring force F_(F)or F_(F1) -F_(F2) does not have a linear, but rather a progressive ordegressive or s-shaped course over the path of the separating piston 22.In the instance of an s-shaped course, the characteristic line is eitherinitially degressive and subsequently progressive or initiallyprogressive and subsequently degressive. In other words, the effectivespring element-which possibly is realized by "combining" several springelements--has a spring stiffness c (force change per path unit) thatchanges over the entire path; the relation c=ΔF/Δs applies. Thisparticularly advantageous measure can be explained as follows.

The natural frequency f of a suspension system depends on the mass m dueto the relation ##EQU1## Consequently, a load change in a motor vehiclewould also cause a detuning of the suspension due to the change of thenatural frequency f. According to the invention, the spring stiffness cthat changes in dependence on the path is selected such that--dependingon the respective instance--the natural frequency f remains at leastapproximately constant or is changed intentionally in order to beoptimally adapted to the respective load.

A few variations of the suspension system according to the invention aredescribed below.

In the embodiments according to FIGS. 1, 2 and 4, the annular chamber 16of the strut 2 is hydraulically connected with the cylinder chamber 14,so that the hydraulic pressure p_(h) is also present at this location.The pressure surface of the piston 10 which is decisive for generatingthe carrying force is obtained from the difference of the two opposingpiston surfaces; the effective pressure surface consequently correspondswith the cross section of the piston rod 12. In the embodimentsaccording to FIGS. 1 and 2, the piston 10 is provided with throughpassages 52 in order to realize this hydraulic connection, with thehydraulic medium being able to flow back and forth between the chambers14 and 16 via said through passages. FIG. 4 shows an alternativesolution in which the annular chamber 16 is connected with the cylinderchamber 14 via an external connection 54. In these embodiments, a pistonseal for the piston 10 is no longer required.

In the embodiment according to FIG. 3, the annular chamber 16 of thestrut 2 is connected with the atmosphere via a ventilation opening 56 ofthe cylinder 8, so that this side of the piston is practicallyunpressurized (atmospheric pressure). Due to this design, the entiresurface of the piston 10 which faces the cylinder chamber 14 is decisivefor generating the carrying force. This measure makes it possible toconstruct the strut 2 in a very compact fashion, also for very heavyloads. Consequently, this embodiment is particularly suitable for motortrucks.

It is also advantageous if the flow of the hydraulic medium between thestrut 2 and the piston-type accumulator 6 is dampened, in particularduring the rebound travel of the strut 2. For this purpose, it ispractical to arrange a damping valve 58 that, in particular, may becontrolled or regulated in dependence on the load in the hydraulicconnection 4 between the strut 2 and the piston-type accumulator (16).Alternatively, a cut-off valve--that, however, is not illustrated in thefigures--may be arranged in this hydraulic connection 4, with saidcut-off valve being able to "decouple" the accumulator 6 from the strut2 such that a blocking of the deflection and rebound movements of thestrut can be attained.

It is advantageous if the strut 2 or its cylinder chamber 14 isconnected with a hydraulic leveling device 60 that in the embodimentshown consists of two on-off valves 62, 64. The intake side of theseon-off valves is connected with a pressure line P as well as a tank lineT, with the output side of said valves being connected with the cylinderchamber 14, so that the latter may be selectively connected with thepressure line P or the tank line T. This measure makes it possible toadjust the level of the strut 2 by introducing or removing the hydraulicmedium. This level adjustment may also be carried out automatically, bymeans of suitable level sensors that acquire the respective position ofthe strut 2--e.g., by measuring the distance between the axle and theframe.

For reasons of completeness, it should be mentioned that the piston-typeaccumulator 6 is, due to the separating piston rod 40 that is connectedwith the separating piston 22, basically constructed as a pressuretransducer in such a way that the pneumatic pressure p_(p) is "usually"always less intense than the hydraulic pressure p_(h). However, thispressure difference is influenced respectively by charging theseparating piston 22 with the supplementary spring force F_(F) accordingto the invention. Depending on the supplementary spring characteristics,this can lead to the pneumatic pressure becoming slightly more intensethan the pneumatic pressure beginning at a certain load condition.However, this can be easily prevented by means of a suitable design.

One advantageous development of the piston-type accumulator 6 isdescribed below. In this case, the separating piston 22 is provided witha closure element 66 on the side of the accumulator chamber 24, withsaid closure element closing a hydraulic connection 70 that exits intothe accumulator chamber 24 if the separating piston 22 is displaced intoa cut-off position in the direction of a final limit stop 68 on the sideof the accumulator chamber, namely in such a way that a closed pressurechamber (not illustrated in the figures) that has the residual volume ofthe accumulator chamber 24 is formed. In this case, the closure element66 is constructed elastically or arranged such that it may be movedelastically relative to the separating piston 22 in such a way that theseparating piston 22 can be moved at least slightly beyond the cut-offposition in the direction of the final limit stop 68 against the forceof a spring. This design has the purpose of insuring a hermetic seal ofthe spring chamber against the discharge of the compressible mediumunder all possible operating conditions. Such leaks in particular canoccur if the accumulator chamber is essentially unpressurized and theseparating piston adjoins the final limit stop on the side of theaccumulator chamber (final rebound position) due to the pneumaticpressure. This can, for example, occur if the spring chamber is alreadyfilled with a certain pneumatic prestress pressure while the accumulatorchamber is not yet hydraulically pressurized during the manufacture ofthe piston-type accumulator.

This critical condition also occurs if the strut 2 is entirelyalleviated from all loads. In these instances, the pneumatic pressure issubstantially more intense than the pressure inside of the accumulatorchamber, which is the reason why the compressible medium could bedischarged via the separating piston seal and the accumulator chamber.Due to the preferred closure element 66, the separating piston 22 bracesitself on a hydraulic medium (hydraulic cushion) that is enclosed in thepressure chamber formed in the cut-off position, i.e., before theseparating piston reaches the mechanical final limit stop 68 on the sideof the accumulator chamber, so that a corresponding counter pressure isalways adjusted automatically on the side of the accumulator chamber orthe pressure chamber due to the fact that the separating piston 22 isstill able to carry out an axial movement in this position and saidseparating piston is charged with the pneumatic pressure. This counterpressure is advantageously adapted automatically if the pneumaticpressure changes, e.g., due to the temperature. This measure insuresthat the separating piston seal is always charged with pressures thatare adapted to one another on both sides, so that said separating pistonseal never needs to provide a seal against the full pneumatic pressure.Additional constructive details regarding the closure element 66 arecontained in the publication German Patent No. U 9,012,936, to which werefer in its entirety at this point.

One additional and particularly advantageous development of theinvention is described below with reference to FIGS. 6 and 7. Thisdevelopment pertains to those embodiments in which the separating piston22 is charged by at least one mechanical spring element 28 via theseparating piston rod 30.

In piston-type accumulators for hydropneumatic suspension systems, it isvery important that the separating piston can be moved very easily,i.e., at very minute pressure changes, in order to insure an optimalfunction of the suspension system. In order to always insure thismovability of the separating piston, the spring element 28 is, accordingto the invention, braced on at least one end via an abutment elementthat is connected with the accumulator cylinder 20 or the separatingpiston rod 30 in a pivoting fashion in such a way that, if the effectiveaxes of the spring element and the accumulator cylinder are offset dueto a relative movement between the abutment element and the cylinder orthe piston rod, an automatic adaptation of the effective axes takesplace.

The aforementioned measure according to the invention is based on theidea that the effective axis (axis of the direction of force) frequentlydeviates from the effective axis (moving axis) of the piston-typeaccumulator due to non-parallelism, namely due to the manufacture and/orthe design of the springs, primarily mechanically acting helicalsprings. This can, for example, be caused by the fact that a helicalpressure spring usually "bulges" from its effective axis when beingcompressed such that an altered direction of force is obtained.Consequently, it is possible that the spring does not only charge thepiston rod in its axial direction, but that a transverse force componentis created that could tilt the piston rod so intensely that a movementof the piston is substantially impaired due to the increased friction oreven entirely impossible in severe instances.

The present invention eliminates this disadvantageous effect with verysimple constructive means because each transverse force generated by thespring causes a corresponding and automatic relative pivoting of therespective abutment element until the effective axis of the piston-typeaccumulator coincides with the effective axis of the spring. In thiscase, the piston rod is advantageously charged with only one exactlyaxially directed force, so that friction or jamming caused by a tiltingof the piston is eliminated.

FIGS. 6 and 7 show in detail that one end of the spring element 28 isbraced on the accumulator cylinder 20 and the other end of said springelement is braced on the free end of the separating piston rod 30. Inthe embodiments shown, the spring element 28 is formed by a helicalspring (pressure or tension spring) 80 that is wound of spring wire.This spring 80 essentially encloses the accumulator cylinder 20 and theseparating piston rod 30 and is prestressed between a first abutmentelement 82 that is fastened onto the free end of the separating pistonrod 30 and preferably has the shape of a disk and a second abutmentelement 84 that is arranged within the region of the axially opposingclosed end of the accumulator cylinder 20.

In "normal instances," the effective axes of the spring element 28 (axisof the spring force) and the piston-type accumulator 6 (moving axis ofthe separating piston 22 and the separating piston rod 30) shouldcoincide as shown in the figures, which is the reason why only onecommon effective axis 86 is shown in FIGS. 6 and 7. In reality, thesetwo effective axes frequently deviate from one another.

Consequently, it is proposed that the spring 80 in the preferredembodiments of the invention directly engages on the accumulatorcylinder 20 on the side of the cylinder, which is the reason why thesecond abutment element 84--that preferably is part of a cylinderholding element 88--is connected in a pivoting fashion with theaccumulator cylinder 20 in such a way that an automatic relativepivoting movement (compare with the double arrow 90) takes place if theeffective axis of the cylinder and the one of the spring element 28deviate from one another until both effective axes 86 coincide or are atleast aligned parallel to one another.

If the effective axes are offset to one another, the relative pivotingmovement according to the invention advantageously takes place betweenthe accumulator cylinder 20 and the second abutment element 84 for thespring 80, so that an adaptation of the effective axis is carried out bychanging the inclination (angle to the effective axis of the cylinder)of a supporting plane of the spring 80 that is defined by the abutmentelement 84.

Additionally or alternatively, it is possible to connect the abutmentelement 82 with the separating piston rod 30 in a pivoting fashion onthe side of the piston rod. However, in the examples shown, apractically rigid connection, i.e., a non-pivoting connection, isprovided at this location.

FIGS. 6 and 7 show that the accumulator cylinder 20 is preferablyconnected with the cylinder holding element 88 or the second abutmentelement 84 via a ball joint 92. In this case, the accumulator cylinder20 preferably is connected with the ball 94 and the cylinder mounting 88is provided with a ball receptacle (ball socket) 96.

The measure for "automatically adapting the effective axes" between acylinder and a spring cannot only be applied in piston-typeaccumulators, but also in other piston/cylinder units in which amechanical spring engages between a cylinder and a piston rod; this is,for example, the case with so-called "struts" (shock absorbers).

The invention is not limited to the embodiments that are illustrated inthe figures and were described previously, but rather also includes allembodiments that function identically in the sense of the invention. Itis, in particular, also possible to use other suitable spring elementsfor generating the supplementary spring force or to provide acombination of different spring elements.

The invention also is not limited to the combination of characteristicsdefined in claim 1, but may also be defined by any arbitrary othercombination of certain characteristics of all individual characteristicsdisclosed. This means that any individual characteristic of claim 1 canbasically be omitted or replaced by at least one individualcharacteristic disclosed at a different text portion of the application.To that extent, the claims only represent a first attempt forformulating the invention.

Consequently, the invention also pertains to a suspension system as itis illustrated in FIG. 8. This suspension system is not a hydropneumaticsuspension system, but rather a hydraulic-mechanical suspension systemsince a piston/spring accumulator 6a is used that does not contain apneumatic medium in its spring chamber 26a. In this case, theaccumulator piston 22a is charged with a spring force F that counteractsthe hydraulic force F_(h). In the example shown, this is realized by amechanical spring element 98 that is constructed as a helical pressurespring 100 and arranged inside of the spring chamber 26a of the springaccumulator 6a, with the spring chamber 26a being connected with theatmosphere via a ventilation opening such that atmospheric pressure ispresent in said spring chamber. Alternatively, it is also possible toarrange a tension spring element, e.g., in the accumulator chamber 24,for generating the spring force F.

The spring characteristic of the spring element 98 is adapted to therespective design of the system components in such a way that it isalways insured that the spring force F acts opposite to the hydraulicforce and lies on the order of the hydraulic force that is generated bythe hydraulic pressure present in dependence on the load. This meansthat the spring element 98 has only little stiffness ("weak spring") ifthe accumulator 6a is designed with a small piston surface that ischarged by the hydraulic pressure--and a correspondingly large pistontravel (for accommodating the respective hydraulic mediumdisplaced)--while the spring has a more intense stiffness ("strongspring") if the accumulator is designed with a larger pistonsurface--and a correspondingly shorter piston travel.

The essential advantage of this design can be seen in the fact that thesame superior spring characteristic and consequently an optimal roadfeel can practically be obtained during an empty load and a full load ofthe strut 2. Sealing problems within the region of the accumulatorpractically no longer occur since the ring of the accumulator piston 22aonly needs to seal against the hydraulic pressure. In addition, thepiston-spring accumulator 6a can be designed for particularly longspring travels in a very simple fashion by constructing saidpiston-spring accumulator correspondingly long (primarily a long springchamber 26a and a long spring element 98). Due to the fact that thestrut 2 is designed as a separate component, the spring accumulator 6acan, despite its length, be easily accommodated in a motor vehicle,e.g., arranged horizontally along a part of the frame of the motorvehicle.

I claim:
 1. Hydropneumatic suspension system for motor vehicles,comprising:at least one hydraulic strut (2) that during deflection andrebound movement acts via a hydraulic medium upon at least onehydropneumatic piston-type accumulator (6) that has a separating piston(22) being guided within an accumulator cylinder (20) in a freelymovable fashion, and separating inside of said accumulator cylinder (20)a hydraulic accumulator chamber (24) from a spring chamber (26); saidhydraulic accumulator chamber (24) being connected to said hydraulicstrut (2) via a hydraulic connection (4); said spring chamber (26)containing a compressible medium; a hydraulic pressure (ph) thatoriginates from the accumulator chamber (24) and a pneumatic pressure(pp) that originates from the spring chamber (26) acting upon twoopposite sides of said separating piston (22); at least onesupplementary spring having a supplementary spring force (FF;F1;F2)acting upon the separating piston (22) of the piston-type accumulator(6) in addition to the forces (Fh,Fp) resulting from charging theseparating piston (22) with the hydraulic pressure (ph) and thepneumatic pressure (pp); the separating piston (22) of the piston-typeaccumulator (6) being connected with a separating piston rod (30)extending through the hydraulic accumulator chamber (24) and out of theaccumulator cylinder (20) in a sealed fashion; and the supplementaryspring force (FF;FF1;FF2) being generated by at least one spring element(34;36) comprising a piston/cylinder unit (38) located outside of thepiston-type accumulator (6) and having at least one pressure chamber(40,42) inside of which is a medium exerting an elastic prestresspressure (p_(v1),p_(v2)) directly operatively associated with theseparating piston rod (30) to apply the supplementary spring forteacting on the separating piston (22) within the accumulator (6), therebyreducing the volume of the compressible medium required for a particularload ratio of the strut.
 2. Suspension system according to claim 1,characterized by the fact that the supplementary spring force (FF; FF1)charges the separating piston (22) in the direction of the hydraulicforce (Fh) that results from the hydraulic pressure, namely in such away that the supplementary spring force is more intense at a lowhydraulic pressure (ph) than at a high hydraulic pressure (ph). 3.Suspension system according to claim 1, characterized by the fact thatthe supplementary spring force (FF;FF2) charges the separating piston(22) in the direction of the pneumatic force (Fp) that results from thepneumatic pressure, namely in such a way that the supplementary springforce is less intense at a low hydraulic pressure (ph) than at a highhydraulic pressure (ph).
 4. Suspension system according to claim 1,characterized by the fact that the pressure chamber(s) (40,42) of thepiston/cylinder unit (38) is filled with a compressible medium that ispressurized with the elastic prestress pressure (pv1,pv2).
 5. Suspensionsystem according to claim 1, characterized by the fact that theintensity of the supplementary spring force (FF;FF1 ;FF2) can be alteredvia adjusting means.
 6. Suspension system according to claim 1,characterized by the fact that the supplementary spring force (FF;FF1;FF2) does not have a linear course over the path of the separatingpiston (22), but rather has one of a progressive or degressive orS-shaped course.
 7. Suspension system according to claim 1,characterized by the fact that the strut (2) comprises a cylinderchamber. (14) that is hydraulically connected with the pistonaccumulator (6) as well as an annular chamber (16) that encloses apiston rod (12) and is separated from the aforementioned cylinderchamber by means of a piston (10), with the annular chamber (16) beingeither hydraulically connected with the cylinder chamber (14) or withthe atmosphere via a ventilation opening (56).
 8. Suspension systemaccording to claim 1, characterized by the fact that a damping valve(58) that is adjustable in dependence on the load is arranged in ahydraulic connection (4) between the strut (2) and the piston-typeaccumulator (6).
 9. Suspension system according to claim 1,characterized by the fact that the strut (2) is selectively connectedwith a hydraulic pressure line (P) or a tank return line (T) via aleveling device (60).
 10. The suspension system according to claim 1,wherein:the spring element (36) comprises a gas spring located partiallywithin the hydraulic accumulator chamber (24) and having a piston (44)operatively associated with the separating piston rod (30).
 11. Thesuspension system as in claim 10, wherein:the gas spring comprises twochambers separated by the piston (44) operatively associated with thepiston rod, and one of said chambers is filled with a pneumatic medium.